The present invention relates generally to improved power system communications, and more particularly to apparatus capable of simultaneously transmitting and receiving multiple multiplexed digital data signals both at high rates and over long distances through power lines and power line transformers, including AC, DC, coaxial cables, and twisted pair lines.
“Power-line Carriers” are well known in the field of power system communications. The principal elements of such power-line carriers are transmitting and receiving terminals, which include one or more line traps, one or more coupling capacitors, and tuning and coupling equipment. Detailed information regarding the description and typical composition of conventional power-line carriers may be found in Fundamentals Handbook of Electrical and Computer Engineering Volume II: Communication Control Devices and Systems, John Wiley & Sons, 1983, pp 617-627, the contents of which are incorporated herein by reference. A significant problem associated with prior art power-line carriers is their requirement for one or more line traps, one or more capacitors, one or more coupling transformers or carrier frequency hybrid circuits and frequency connection cables.
All traditional couplers incorporate a ferrite or iron core transformer which causes signal distortion due to the non-linear phase characteristic of the transfer function between the transmit coupler and the receive coupler. The distortion is created by the presence of magnetic core material which exhibits hysteresis. For distribution power-line carriers, the distortion is particularly severe because the signal must propagate through at least three such non-linear devices, the distribution transformer and two power-line couplers, that use ferrite core transformers. The distortion caused by these non-linear devices leads to envelope delay distortion, which limits communication speeds.
The major shortcoming of previous designs resulted from the use of ferrite or iron core transformers in the signal couplers. The primary winding inductance, L1, is altered to some unknown value due to the non-linearity of the core. This results in a mistuning of the desired carrier frequency. Also, the impedance of the primary winding at the desired carrier frequency is no longer matching the power line characteristic impedance. In recognition of this fact, other designs attempt to merely couple a signal onto a power line with a low transceiver input impedance by using a large coupling capacitor (approx. 0.5 uF). This results in a significant coupling loss of up to 20 dB at the carrier frequency.
U.S. Pat. No. 6,407,987 (Abraham) discloses a novel phase shift linear power, phone, twisted pair, and coaxial line coupler for both transmission and reception. The phase shift linear coupler comprises a novel air-core or dielectric core transformer which can be used for phone line, coaxial, LAN and power line communication through power line transformers. The phase shift linear coupler further comprises an associated coupling capacitor network in order to achieve resistive matching to approximately the lowest known value of the line characteristic impedance and to maximize stable signal transmission onto the line. This resonance effectively creates a band pass filter at carrier frequency. U.S. Pat. No. 6,407,987 is incorporated herein by reference in its entirety.
The designs described in U.S. Pat. No. 6,407,987 solved many of the problems of previous designs, which used ferrite or iron couplers that resonated with the power line characteristic impedance, resulting in notches, suck outs and non-linear media for communications over various lines such as power lines. The phase shift linear coupler in U.S. Pat. No. 6,407,987 does not have notches at the communications bandwidth, allowing linear communication over a very wide range of frequencies.
There is still a need, however, for a power line communications system capable of simultaneously transmitting and receiving multiple digital data signals using higher frequencies (e.g., 200 Mhz-500 GHz), thereby permitting communication at high rates using wide bandwidths and over long distances through power lines and power line transformers, including AC, DC, coaxial cables, and twisted pair lines.